1. Field of the Invention
The invention relates to a method for determining the flux vector of a rotating field machine on which a stator current vector is impressed with a frequency changer. The invention further relates to the application of the method to the operation of the rotating-field machine, a device for determining the flux vector of the rotating-field machine and a method for operating this device.
2. Description of Related Art
Field oriented controls are known for the highly dynamic operation of rotating-field machines that use two mutually independent reference values. These reference values determine the component of the stator current vector parallel and perpendicular to the flux vector in a coordinate system rotating synchronously with the flux. A control uses the direction angle of the flux vector to transform the coordinate system of the machine stator winding while at rest. This transformation is connected with a corresponding stator current control and furnishes the setting signals for a frequency converter. The frequency converter then impresses the three phase winding on the stator winding which corresponds to the reference vector formed from the reference values of the components.
The information about the flux vector required for this field-oriented control, including at least its direction, can be calculated from the stator voltage and the stator current using a "voltage model". The EMF vector is determined in the stator oriented reference system. A model vector of the flux is then calculated by vectorial integration. The stator voltage assumes only small values at low frequencies. Noise is superimposed on the stator vector with such strength that the EMF vector can be determined with reasonable reliability only at higher frequencies. Furthermore, a null drift of the measuring members and integrators combine with other calculating errors to cause an erroneous d-c component to be calculated into the model vector.
The errors associated with the voltage model can be overcome using the "current model". This latter model measures a value for the rotor position and calculates the flux from the measured stator currents and a parameter for the rotor time constant. It therefore simulates in the machine electrical processes leading to the build-up of the flux. The current model, however, is dynamically inferior to the voltage model, especially at higher frequencies.
West German Patent No. 3 319 350 discloses a combination of both the voltage and current models. The model vector is calculated by the integration of a sum vector formed from the EMF vector and a correction vector. The components of this correction vector are given in a coordinate system oriented toward the model vector in such a manner that the control deviation of the magnitude of a model vector from the reference value of the flux is averaged by the component of the correction vector that is parallel to the model vector. The difference angle between a model vector and a second model vector that is calculated in a current model perpendicular to the model vector is averaged via the component perpendicular to the model vector.
European Patent No. 71, 847 proposes adding "magnetizing current" to the field-parallel component. The "magnetizing current" is an oscillating supplemental reference value that is used to calculate the rotor time constant required in the current model. An oscillating component is then impressed on the stator current vector of the machine in the component parallel to the field. The component of the stator current vector that is perpendicular to the field, the "active current", determines the torque. This perpendicular component is not influenced by the supplemental reference value. The supplemental reference value acts on the true magnetizing current belonging to the true flux vector as well as on the true active current in the event of a misadjustment of the parameter used in the current model for the rotor time constant and a miscalculation of the flux vector connected therewith. The oscillations of the supplemental reference value are also noted in the torque. Multiplication of the measurement value of these torque variations by the time-delayed supplemental reference value followed by subsequent integration forms a correlation signal that is zero only if the model value of the flux coincides with the true flux vector. The correlation signal is therefore used to readjust the rotor time constant of the current model until the correlation signal disappears.
The flux of a synchronous machine also can be calculated if the rotor winding and the field current are included in the current model. However, the current model fails for permanently excited machines. Another decisive disadvantage is the requirement for a complicated mechanical transmitter to determine the rotor position.
West German Patent No. 35 42 941 discloses an apparatus where a high-frequency current is impressed on a stator winding that is partially cross-coupled into the other stator windings. The cause of this cross-coupling is primarily an asymmetry caused by the flux vector of the effective impedances in the machine. The amplitudes of the cross-coupled high frequency signal depend on the different angle between the impressed high frequency current and the field vector. The envelopes of these signals can be used directly as a substitute for the signal of the mechanical transmitter.